This question already has an answer here:
This is a little confusing to see and explain. Let's take this chat message. Someone posted a link to an answer in the Tavern, and that link got oneboxed:
This works fine. There's nothing amiss here.
Then that chat message was, in turn, linked to in another chat room, and it got oneboxed, and so we also saw that original oneboxed link to the answer. At this point though, the link inside goofs out:
A mysterious and undesirable "chat.meta.stackexchange.com//" has appeared. As you can imagine, that URL doesn't work, so it's not some clever forwarding scheme that's been implemented.
(Oneboxing by yet one more level does not appear to exacerbate the problem.)
Probably that undesirable portion of the URL should be thanked for its eagerness to help, and then disinvited from future links.